Function SplitPolygon

Descent3/bsp.cpp:321–449

Tries to split a polygon across a plane

// Tries to split a polygon across a plane
int SplitPolygon(bspplane *plane, bsppolygon *testpoly, bsppolygon **frontpoly, bsppolygon **backpoly) {
  float dists[256], t;
  int numvert, numfront, numback, i, codes[256] = {};
  vector *frontvert[256], *backvert[256], *polyvert[256];
  vector *vertptr1, *vertptr2;
  vector delta, *newvert[256];
  int num_new_verts = 0;

  /* Set up the function. Set all counters, variables, lists etc to
   * zero or NULL, which provides a fall back result, should nothing
   * be changed, and simply means that particular result is to be
   * ignored
   */

  numvert = testpoly->nv;
  numfront = numback = 0;

  /* Now, we shall classify each vertex in the polygon, with both
   * a plane distance, and a classification code. This is done here,
   * as the results are often re-used, which saves time.
   */

  for (i = 0; i < numvert; i++) {
    vertptr1 = &testpoly->verts[i];
    polyvert[i] = vertptr1;
    codes[i] = ClassifyVector(plane, vertptr1);
    dists[i] = plane->a * vertptr1->x + plane->b * vertptr1->y + plane->c * vertptr1->z + plane->d;
  }

  /* We must duplicate the first entry in the numvert+1 slot, so that
   * we can use wraparound to easily check things
   */

  codes[numvert] = codes[0];
  dists[numvert] = dists[0];

  /* Now, the actual splitting work. We must work through each vertex
   * of the polygon, examining it with regard to the plane
   * Where the classification codes differ, we have to split
   */

  for (i = 0; i < numvert; i++) {
    vertptr1 = polyvert[i];

    /* A point is found to be on the plane. This counts for
     * both polygons, as often a vertex may lie along a plane,
     * but will not be splitting it
     */

    if (codes[i] == BSP_ON_PLANE) {
      frontvert[numfront++] = vertptr1;
      backvert[numback++] = vertptr1;
    } else if (codes[i] == BSP_IN_FRONT) {
      /* Simple cases of point being in front or behind
       * the plane. Just insert them to the appropriate
       * lists.
       */
      frontvert[numfront++] = vertptr1;